1. Field of the Invention
The present invention relates to a toner for image forming methods such as electrophotography, electrostatic recording, electrostatic printing and toner jetting, in particular a color toner suitable for oil-less fixation.
2. Related Background Art
Recently, more compact, less weighted, more speedy and more reliable copiers and printers are required in view of space-saving and energy-saving points. As a result, the hardware has become comprised of simpler elements, and higher performance is required for the toner. In other words, without improvement in the toner performance, it is difficult to provide an excellent hardware. Particularly important color toner performance is color reproducibility.
Various methods are employed for full-color copiers and printers. One of the generally employed methods forms a full-color image by forming an electrostatic image on each of the photosensitive members, developing the images with a cyan, magenta, yellow and black toner, and feeding a transfer material between each photosensitive member and a transfer belt to transfer the images to the transfer material in a straight pass. Another method forms a full-color image by winding a transfer material on the transfer member facing the photosensitive member using electrostatic force or mechanical action such as a gripper and by conducting the development/transfer cycles four times.
It is required for these full-color image-forming toners, to be sufficiently mixed with each other before the image is fixed onto a transfer material with heat or pressure in the fixation step, without deteriorating color reproducibility or transparency of overhead projector (OHP) image.
Binder resins for toners include styrene-, polyester- and epoxy-based resin, and polyester resin is more preferable in view of sharp melting and low temperature fixation properties. Recently, use of a mixture of two or more polyester resins different in the softening point has been studied to expand the fixation region. Use of two or more resins will make uniform dispersion of the colorant during the hot melt-kneading step in the toner production more difficult.
Several attempts have been made to improve dispersion of a colorant to solve the above problems. Japanese Unexamined Patent Publication No. H8-15909 discloses preparation of a master batch containing a pigment kneaded beforehand into a binder resin at a high concentration, followed by dilution kneading of the master batch with the same binder resin and a charge-controlling agent or the like. Japanese Unexamined Patent Publication No. H7-295293 discloses an attempt to improve dispersion by using a specific combination of a pigment and a polyester resin.
These patent documents, however, are silent on a color toner containing a polyester binder resin synthesized in the presence of a specific polycondensation catalyst and also containing a release agent.